U.S. Pat. No. 5,106,270 to Goettel et al discloses an air compressor comprised of two low pressure cylinders each of which discharges low pressure air into respective intercoolers to cool such compressed air before it enters a common manifold connection and inlet flange of a high pressure cylinder of the compressor. A single intercooler core design is also available that collectively receives the air discharged from low pressure cylinder heads and, again, cools the air before entering the high pressure head for second stage compression.
The compressor can be driven by an electric motor, as disclosed in the Goettel et al patent, though in times past, compressors in locomotives were driven directly by the diesel engine of the locomotive. In this manner, while the diesel engine was idling, the compressor continued to run, though at the slow idle speed.
Recent compressor designs are operated by electric motors in a stop/start fashion, the compressors being started when pressurized air is needed and stopped when pressurized air is not needed. Such electric motors operate from a voltage generated by an alternator in the locomotive, which alternator is driven by the diesel engine of the locomotive. When diesel engine RPM is low, such as in an idle condition, the alternator produces only a limited amount of electrical power. Such a limited amount of power may be insufficient to operate the compressor motor at a speed sufficient for the compressor to deliver the required amount of compressed air. When this occurs, the air compressor must operate at a speed greater than that at which the motor is capable of operating when supplied by the electrical characteristics of the alternator.
For this reason, compressor motors may have a dual pole, dual speed configuration, i.e., a first group of magnetic poles equal in number to that of the supply voltage alternator are provided such that for low speed operation, the compressor turns at essentially the same speed as the alternator (and the mechanical drive of the diesel engine) less any losses, since the poles of the motor and alternator are equal. However, the compressor should also be able to run faster than engine speed, such as an idle speed, to assure a sufficient compressed air output and to overcome train line losses. If a second group of motor poles is provided that is half that of the alternator, the compressor runs at twice diesel engine/alternator speed. In this manner, the locomotive crew can operate the locomotive at a lower engine speed (to save fuel and reduce engine wear) while, at the same time, produce a sufficient amount of compressed air for brakes and other pneumatically operated devices.
When additional air pressure is called for, the compressor motor is signaled to operate at the higher speed. When this occurs, the compressor is unloaded (exhausted) of air pressure so that the motor can start (transition) under unloaded conditions. When a compressor is unloaded, it rotates freely and thus places a very light load on the motor driving the compressor. If the motor is required to start or transition against a pressure load in the compressor, the rotor of the compressor can appear to the motor to be locked, and can thereby burn out the motor, as the motor draws large amounts of current to overcome the force of compression in the compressor. The compressor rotor includes a crankshaft that operates pistons in the cylinders of the compressor, the pistons being the agent by which the compressed air is formed. It is therefore understandable that with air pressure in the cylinders acting against the pistons and thus against the crankshaft of the compressor, the drive motor of the compressor has a difficult task in rotating the crankshaft.
The time duration for a dual pole configuration motor to transition from its relatively slow speed operation to the doubling high speed operation is on the order of one second. Hence, when the motor changes speed there may still be air pressure in the high pressure head of the compressor, as supplied by the intercooler(s). It is therefore necessary that intercooler air pressure be discharged rapidly so that the compressor motor does not have to start, i.e., change speeds, against a pressure load in the high pressure cylinder of the compressor.